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Defect Thermodynamics and Kinetics in Polyanionic Cathodes: A Theoretical Roadmap for Na-ion based Batteries and Hybrid Supercapacitors
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.ORCID iD: 0000-0003-1265-5912
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, the framework of the density functional theory is employed to study and predict properties of polyanionic cathodes for Na-ion batteries and hybrid supercapacitors. It consists of three main parts as follows:

The first part is primarily dedicated to kröhnkite-type Na2Fe(SO4)22H2O cathode. The major goal is to probe diffusion mechanisms of Na+ ions.  The chemical potentials diagrams for the pentrary compound are determined under thermodynamic equilibrium and are used to calculate pH value for solution-based synthesis. We find that the presence of NaFe facilitates a faster migration and reduces the channel blockage issue. Moreover, the defect concentration can be tuned by controlling the pH condition. We conclude that defects and small hole polarons play a role in ionic and electronic conductivity.

The second part focuses on alluaudite-type Na2+2δFe2-δ(SO4)3 (NFSδ). We unveil the effect of the non-stoichiometry on the thermodynamics, defect nature, and voltage profiles NFSδ with δ = 0, 0.25 and 0.5. The relation between Na ion distribution and energetics is studied and reveals the necessity of using a supercell model. Chemical potential diagrams indicate an inevitable impurity precipitation in all cases, but can be reduced at low δ. Defect formation analysis shows an unlikely formation of channel blockage and can explain the impurity precipitation in experiment. Two types of phase transition are observed after half-desodiation. A higher degree of non-stoichiometry offers an improvement in specific capacity and structural reversibility for NFS0.25 and NFS0.5. The voltage profiles and formation energy reveal the Na intercalation mechanism and strategy to enhance the specific capacity.

The third part is associated with battery-type cathodes used in hybrid supercapacitors, namely the NaMPO4 and MMoO4 (where M is a transition metal). We find that triphylite NaNiPO4 shows a better electrochemical performance as compared to maricite phase due to the merit of intercalation mechanism. A mixed-NaMn1/3Co1/3Ni1/3PO4 is predicted to show faradaic behavior, mainly contributed from the Ni and Mn redox reactions, along with an improved electronic conductivity. Moreover, the effect of M substitution on phase stability, electronic properties and charge transfer is also studied in MMoO4 with M = Mn, Co and Ni. The highest capacitance is predicted for NiMoO4 amongst the others and is attributed to the higher active surface area. To compromise the capacitance and cycling stability, Mn1/3Co1/3Ni1/3MoO4 is synthesized. We predict its crystal structure by using the SQS method. Based on electronic structure, we can identify a source of the improved cycling efficiency and specific capacitance of this mixed compound.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 92
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1796
Keywords [en]
DFT, Energy Materials, Defects, Chemical potentials, Kinetics, Hybrid supercapacitors, Na-ion batteries, Polyanionic cathodes
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-381173ISBN: 978-91-513-0628-5 (print)OAI: oai:DiVA.org:uu-381173DiVA, id: diva2:1302543
Public defence
2019-05-22, Room 80101, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2019-04-29 Created: 2019-04-05 Last updated: 2019-06-18
List of papers
1. Mechanistic study of Na-ion diffusion and small polaron formation in Kröhnkite Na2Fe(SO4)2·2H2O based cathode materials
Open this publication in new window or tab >>Mechanistic study of Na-ion diffusion and small polaron formation in Kröhnkite Na2Fe(SO4)2·2H2O based cathode materials
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2017 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 5, no 41, p. 21726-21739Article in journal (Refereed) Published
Abstract [en]

Kröhnkite-type Na2Fe(SO4)2·2H2O mineral is a sustainable and promising polyanionic cathode that has been experimentally found to offer a high redox potential (3.25 V vs. Na/Na+) along with fast-ion diffusion and high reversibility. Owing to the structural complexity, Na+ diffusion was assumed to occur along a convoluted channel along the b-axis. However, theoretical work related to this material still appears missing to support that statement. In this work, DFT+U calculations have been performed with the primary aim to unveil the Na+ diffusion mechanism in this material. The electronic structure and charge transfer are also envisaged to reveal evidence of Fe2+/3+ redox reaction and a vital role of structural H2O. Based on formation energies of this material with varied Na concentration, a calculated voltage profile is determined to show two voltage plateaus at 4.81 and 3.51 V, corresponding to experimental results. Nudged elastic band calculation reveals that Na+ diffusion is primarily occuring in the [01] direction with a moderate ionic mobility due to the structural distortion induced during migration, suggesting the possibility of defect-assisted diffusion. Intriguingly, the formation of small hole polarons is first observed, and could play a key role in the electronic conduction of this material.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-340686 (URN)10.1039/c7ta04508e (DOI)000413734800014 ()
Funder
Swedish Research CouncilCarl Tryggers foundation StandUp
Note

Title in WoS: Mechanistic study of Na-ion diffusion and small polaron formation in Krohnkite Na2Fe(SO4)(2)center dot 2H(2)O based cathode materials

Available from: 2018-02-02 Created: 2018-02-02 Last updated: 2019-04-05Bibliographically approved
2. Defect formations and pH-dependent kinetics in krohnkite Na2Fe (SO4)2·2H2O based cathode for sodium-ion batteries: Resembling synthesis conditions through chemical potential landscape
Open this publication in new window or tab >>Defect formations and pH-dependent kinetics in krohnkite Na2Fe (SO4)2·2H2O based cathode for sodium-ion batteries: Resembling synthesis conditions through chemical potential landscape
2019 (English)In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 55, p. 123-134Article in journal (Refereed) Published
Abstract [en]

Thermodynamics and kinetics of intrinsic point defects in Na2Fe(SO4)(2)center dot 2H(2)O, a high-voltage cathode for Na-ion batteries, are studied by means of first-principles density functional theory. Electronic structures of charged defects are calculated to study their influences towards electronic and electrochemical properties as well as to probe hole polaron formation. As defect formation energy strongly depends on atomic chemical potentials, we initiate a systematic approach to determine their valid ranges for the pentrary Na-Fe-S-O-H compound under thermodynamic equilibria and correlate them with approximated pH parameters in solution-based synthesis. Given chemical potential landscape and formation energy, we find that Fe-Na(1+), V-Na(1-,0) and Na-Fe(1-,0) are dominant and their concentrations could be manipulated through pH condition and oxygen content in the precursor solution. It is predicted that the channel blockage due to Fe-Na would appear under strong acidic growth condition but could be diminished under weak acidic condition (4.7 <= pH <= 5.6) where Na-Fe facilitates a faster migration between each diffusion channel. Our results do not only explain the origin of intercalation mechanism and improved electronic conduction, but also demonstrates the pH influence towards conductivities in the cathode material.

Keywords
Chemical potentials, Defects, DFT, Diffusions, Sodium-ion batteries
National Category
Materials Chemistry Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-374115 (URN)10.1016/j.nanoen.2018.10.038 (DOI)000454636200012 ()
Funder
Swedish Research CouncilSwedish Research CouncilCarl Tryggers foundation
Available from: 2019-01-23 Created: 2019-01-23 Last updated: 2019-04-05Bibliographically approved
3. Defect Thermodynamics in Nonstoichiometric Alluaudite-Based Polyanionic Materials for Na-Ion Batteries
Open this publication in new window or tab >>Defect Thermodynamics in Nonstoichiometric Alluaudite-Based Polyanionic Materials for Na-Ion Batteries
2019 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252Article in journal, Letter (Refereed) Published
Abstract [en]

Sodium iron sulfate in the form of alluaudite Na2+2xFe2-x(SO4)3 (or NFSx) has emerged as one of the most promising cathodes for Na-ion batteries due to its highest Fe2+/3+ redox potential, low-cost, sustainability and high rate capability. Unlike most of the other cathodes, NFSx  generally crystalizes in its non-stoichiometric form with partial Na substitution for Fe sites and contains a small amount of impurities. However,  profound explanations behind this inherent behavior including others, like phase stability, configurational structure and defect formation are still  ambiguous. We have therefore performed  first principles calculations combined with a random swapping method to determine the minimum energy configurations of NFSx (with x = 0, 0.25 and 0.5) and find a correlation between the Na distribution pattern and energetics in which the site preference for Na+ ion is in a sequence of Na4 > Na1 > Na2 > Na3. Our result points out that the non-stoichiometry cannot be properly described under the framework of primitive structures. Moreover, we investigated phase stability diagrams and defect formations based on thermodynamic criteria. Our predicted phase diagrams can explain the inevitable impurity precipitation, which can be reduced as x diminishes. Defect formation analysis indicates an unlikely formation of channel blockage and identifies the dominant formation of FeNa+VNa and Nai+NaFe complexes. While the former can become spontaneous in a Na-deficient environment, the latter occurs mainly in NFS0 and accommodates the presence of non-stoichiometry.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-381171 (URN)
Available from: 2019-04-05 Created: 2019-04-05 Last updated: 2019-08-16
4. Mapping Sodium Intercalation Mechanism, Electrochemical Properties and Structural Evolution in Non-stoichiometric Alluaudite Na2+2δFe2-δ(SO4)3 Cathode Materials
Open this publication in new window or tab >>Mapping Sodium Intercalation Mechanism, Electrochemical Properties and Structural Evolution in Non-stoichiometric Alluaudite Na2+2δFe2-δ(SO4)3 Cathode Materials
2019 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 7, p. 17446-17455Article in journal (Refereed) Published
Abstract [en]

In the scientific advancement of future cathode materials, alluaudite sodium iron sulfate Na2+2δFe2−δ(SO4)3 (NxFyS) has emerged as one of the most promising candidates for sustainable sodium-ion batteries due to its high Fe2+/3+ redox potential (3.8 V vs.Na/Na+), low cost, and high rate capability. Usually, this material occurs in a non-stoichiometric form with partial Na+ substitutions on Fe sites, where δ is close to 0.25 (N2.5F1.75S) depending on the synthesis conditions. While many contemporary works have primarily been directed to study this non-stoichiometric compound, our previous theoretical prediction unveiled the possibility to synthesize stoichiometric alluaudite (N2F2S), which is expected to deliver higher specific capacity (∼120 mA h g−1) as compared to the non-stoichiometric derivatives. This provokes curiosity toward the non-stoichiometric effect on the electrochemical activities and sodium intercalation mechanism in alluaudite materials. In this work, we therefore perform rigorous first-principles calculations to study the structural evolution, electrochemical behavior, and voltage profile of NxFyS with y = 2, 1.75, and 1.5. We reveal the likelihood of two phase transitions after half desodiation process, whereas the probability is reduced with a higher degree of non-stoichiometry, suggesting improvement in the structural reversibility for N2.5F1.75S and N3F1.5S. The prediction of the voltage profiles shows the benefit of non-stoichiometry in enhancing the specific capacity and identifies the structural rearrangement of Fe2O10 dimers as the hidden reason behind the irreversible sharp peak experimentally observed in differential galvanostatic profiles.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-381172 (URN)10.1039/c9ta03930a (DOI)000476913600026 ()
Funder
Swedish Research CouncilStandUpCarl Tryggers foundation
Available from: 2019-04-05 Created: 2019-04-05 Last updated: 2019-08-22Bibliographically approved
5. Current computational trends in polyanionic cathode materials for Li and Na batteries
Open this publication in new window or tab >>Current computational trends in polyanionic cathode materials for Li and Na batteries
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2018 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, no 28, article id 283003Article, review/survey (Refereed) Published
Abstract [en]

A long-standing effort has been devoted for the development of high energy density cathodes both for Li-and Na-ion batteries (LIBs and SIBs). The scientific communities in battery research primarily divide the Li- and Na-ion cathode materials into two categories: layered oxides and polyanionic compounds. Researchers are trying to improve the energy density of such materials through materials screening by mixing the transition metals or changing the concentration of Li or Na in the polyanionic compounds. Due to the fact that there is more stability in the polyanionic frameworks, batteries based on these materials mostly provide a prolonged cycling life as compared to the layered oxide materials. Nevertheless, the bottleneck for such compounds is the weight penalty from polyanionic groups that results into the lower capacity. The anion engineering could be considered as an essential way out to design such polyanionic compounds to resolve this issue and to fetch improved cathode performance. In this topical review we present a systematic overview of the polyanionic cathode materials used for LIBs and SIBs. We will also present the computational methodologies that have become significantly relevant for battery research. We will make an attempt to provide the theoretical insight with a current development in sulfate (SO4), silicate (SiO4) and phosphate (PO4) based cathode materials for LIBs and SIBs. We will end this topical review with the future outlook, that will consist of the next generation organic electrode materials, mainly based on conjugated carbonyl compounds.

Keywords
Li-ion batteries, Na-ion batteries, energy storage, polyanionic cathode materials, organic cathode materials, DFT investigation
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-368909 (URN)10.1088/1361-648X/aac62d (DOI)000436088400001 ()29932053 (PubMedID)
Funder
Swedish Research CouncilCarl Tryggers foundation StandUp
Available from: 2018-12-10 Created: 2018-12-10 Last updated: 2019-04-05Bibliographically approved
6. Effect of Transition Metal Cations on Stability Enhancement for Molybdate-Based Hybrid Supercapacitor
Open this publication in new window or tab >>Effect of Transition Metal Cations on Stability Enhancement for Molybdate-Based Hybrid Supercapacitor
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2017 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 9, no 21, p. 17977-17991Article in journal (Refereed) Published
Abstract [en]

The race for better electrochemical energy storage systems has prompted examination of the stability in the molybdate framework (MMoO4; M = Mn, Co, or Ni), based on a range of transition metal cations from both computational and experimental approaches. Molybdate materials synthesized:with controlled nano scale morphologies (such as nanorods, agglomerated nanostructures, and nanoneedles for Mn, Co, and Ni elements, respectively) have been used as a cathode in hybrid energy storage systems. The computational and experimental data confirms that the MnMoO4 crystallized in beta-form with alpha-MnMoO4 type whereas Co and Ni cations crystallized in alpha-form with alpha-CoMoO4 type structure. Among the various transition metal Cations studied, hybrid device comprising NiMoO4 vs activated carbon exhibited excellent electrochemical performance having the specific capacitance 82 F g(-1) at a current density of 0.1 A g(-1) but the cycling Stability, needed to be significantly improved. The specific capacitance of the NiMoO4 electrode material is shown to be directly related to the surface area of the electrode/electrolyte interface, but the CoMoO4 and MnMoO4 favored a bulk formation that could be suitable for structural stability. The useful insights from the electronic structure analysis and effective mass have been provided to: demonstrate the role of cations in the molybdate structure and its influence in electrochemical energy storage. With improved cycling stability, NiMoO4 can be suitable for renewable energy storage. Overall, this study will enable the development of next generation molybdate materials with multiple cation substitution resulting,in better cycling stability and higher specific capacitance.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2017
Keywords
capacitor, molybdate, stability, structural, computation, theoretical
National Category
Nano Technology
Identifiers
urn:nbn:se:uu:diva-327145 (URN)10.1021/acsami.7b03836 (DOI)000402691600035 ()28481523 (PubMedID)
Funder
Swedish Research CouncilStandUp
Available from: 2017-08-28 Created: 2017-08-28 Last updated: 2019-04-05Bibliographically approved
7. A combined theoretical and experimental approach of a new ternary metal oxide in molybdate composite for hybrid energy storage capacitors
Open this publication in new window or tab >>A combined theoretical and experimental approach of a new ternary metal oxide in molybdate composite for hybrid energy storage capacitors
2018 (English)In: APL MATERIALS, ISSN 2166-532X, Vol. 6, no 4, article id 047701Article in journal (Refereed) Published
Abstract [en]

Sustainable energy sources require an efficient energy storage system possessing excellent electrochemical properties. The better understanding of possible crystal configurations and the development of a new ternary metal oxide in molybdate composite as an electrode for hybrid capacitors can lead to an efficient energy storage system. Here, we reported a new ternary metal oxide in molybdate composite [(Mn1/3Co1/3Ni1/3)MoO4] prepared by simple combustion synthesis with an extended voltage window (1.8 V vs. Carbon) resulting in excellent specific capacity 35 C g−1 (58 F g−1) and energy density (50 Wh kg−1 at 500 W kg−1) for a two electrode system in an aqueous NaOH electrolyte. The binding energies measured for Mn, Co, and Ni 2p are consistent with the literature, and with the metal ions being present as M(II), implying that the oxidation states of the transition metals are unchanged. The experimental findings are correlated well through density functional theory based electronic structure calculations. Our reported work on the ternary metal oxide studies (Mn1/3Co1/3Ni1/3)MoO4 suggests that will be an added value to the materials for energy storage.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-355692 (URN)10.1063/1.4994750 (DOI)000431141500010 ()
Available from: 2018-07-04 Created: 2018-07-04 Last updated: 2019-04-05Bibliographically approved
8. Synthesis, and crystal and electronic structure of sodium metal phosphate for use as a hybrid capacitor in non-aqueous electrolyte
Open this publication in new window or tab >>Synthesis, and crystal and electronic structure of sodium metal phosphate for use as a hybrid capacitor in non-aqueous electrolyte
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2015 (English)In: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, Vol. 44, no 46, p. 20108-20120Article in journal (Refereed) Published
Abstract [en]

Energy storage devices based on sodium have been considered as an alternative to traditional lithium based systems because of the natural abundance, cost effectiveness and low environmental impact of sodium. Their synthesis, and crystal and electronic properties have been discussed, because of the importance of electronic conductivity in supercapacitors for high rate applications. The density of states of a mixed sodium transition metal phosphate (maricite, NaMn1/3Co1/3Ni1/3PO4) has been determined with the ab initio generalized gradient approximation (GGA)+Hubbard term (U) method. The computed results for the mixed maricite are compared with the band gap of the parent NaFePO4 and the electrochemical experimental results are in good agreement. A mixed sodium transition metal phosphate served as an active electrode material for a hybrid supercapacitor. The hybrid device (maricite versus carbon) in a nonaqueous electrolyte shows redox peaks in the cyclic voltammograms and asymmetric profiles in the charge-discharge curves while exhibiting a specific capacitance of 40 F g(-1) and these processes are found to be quasi-reversible. After long term cycling, the device exhibits excellent capacity retention (95%) and coulombic efficiency (92%). The presence of carbon and the nanocomposite morphology, identified through X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) studies, ensures the high rate capability while offering possibilities to develop new cathode materials for sodium hybrid devices.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-270648 (URN)10.1039/c5dt03394b (DOI)000365402100045 ()26530639 (PubMedID)
Funder
Carl Tryggers foundation Swedish Research CouncilSwedish Energy Agency
Available from: 2016-01-01 Created: 2016-01-01 Last updated: 2019-04-05Bibliographically approved
9. Synthesis, structural and electrochemical properties of sodium nickel phosphate for energy storage devices
Open this publication in new window or tab >>Synthesis, structural and electrochemical properties of sodium nickel phosphate for energy storage devices
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2016 (English)In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 8, no 21, p. 11291-11305Article in journal (Refereed) Published
Abstract [en]

Electrochemical energy production and storage at large scale and low cost, is a critical bottleneck in renewable energy systems. Oxides and lithium transition metal phosphates have been researched for over two decades and many technologies based on them exist. Much less work has been done investigating the use of sodium phosphates for energy storage. In this work, the synthesis of sodium nickel phosphate at different temperatures is performed and its performance evaluated for supercapacitor applications. The electronic properties of polycrystalline NaNiPO4 polymorphs, triphylite and maricite, t- and m-NaNiPO4 are calculated by means of first-principle calculations based on spin-polarized Density Functional Theory (DFT). The structure and morphology of the polymorphs were characterized and validated experimentally and it is shown that the sodium nickel phosphate (NaNiPO4) exists in two different forms (triphylite and maricite), depending on the synthetic temperature (300-550 degrees C). The as-prepared and triphylite forms of NaNiPO4 vs. activated carbon in 2 M NaOH exhibit the maximum specific capacitance of 125 F g(-1) and 85 F g(-1) respectively, at 1 A g(-1); both having excellent cycling stability with retention of 99% capacity up to 2000 cycles. The maricite form showed 70 F g(-1) with a significant drop in capacity after just 50 cycles. These results reveal that the synthesized triphylite showed a high performance energy density of 44 Wh kg(-1) which is attributed to the hierarchical structure of the porous NaNiPO4 nanosheets. At a higher temperature (>400 degrees C) the maricite form of NaNiPO4 possesses a nanoplate-like (coarse and blocky) structure with a large skewing at the intermediate frequency that is not tolerant of cycling. Computed results for the sodium nickel phosphate polymorphs and the electrochemical experimental results are in good agreement.

National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-303158 (URN)10.1039/c6nr01179a (DOI)000380888200053 ()27189034 (PubMedID)
External cooperation:
Available from: 2016-09-15 Created: 2016-09-15 Last updated: 2019-04-05Bibliographically approved

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